Use bundle check for gem install (closes: #93)

Updated and everything went good (initial/re-run), thanks @igalic for pointing this !

`.git_setup_done` was removed in profit to `bundle check` cmd (see log after)
`.gitlab_setup_done` is keept in order to not re-run initial setup.

(Travis and functional testing with vagrant was happy)

```bash
git@gitlab:~/gitlab$ bundle check
The Gemfile's dependencies are satisfied
git@gitlab:~/gitlab$ echo $?
0
git@gitlab:/tmp/gitlabhq$ bundle check
The following gems are missing
...
git@gitlab:/tmp/gitlabhq$ echo $?
1
```